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A new hypothesis on the origin of 'junk' DNA

BLOOMINGTON, Ind. -- The explosion of "junk" DNA in animals, plants and fungi may be the simple result of their ancestors' reduced population sizes, according to a new hypothesis proposed by Indiana University Bloomington and University of Oregon scientists in the Nov. 21 issue of Science.

The hypothesis explains a mysterious genetic difference between bacteria and eukaryotes, a giant group of organisms that includes animals, plants, fungi, algae and other protists. Bacteria tend to have extremely lean genomes; their genes barely fit into them, without much genetic material left over. Eukaryotic genomes are a complex mixture of useful genes and useless ("junk") DNA jammed haphazardly between genes and even within them.

"The evolution of genomic complexity is inevitable," said IUB biologist Michael Lynch, who led the study. "It's just that in bacteria, there is a pressure against it -- natural selection -- which works more efficiently when population sizes are big. Eukaryotes have much smaller population sizes compared to bacteria, and we believe this is the main reason junk DNA sequences are still with us."

Junk DNA dominates eukaryotic chromosomes. The chromosomal space taken up by just 30 human genes and the DNA within and between those genes could easily accommodate whole bacterial genomes containing 3,000 or 4,000 genes, Lynch said. While some of what geneticists have called junk DNA is turning out to be not so junky after all, Lynch said a substantial fraction of such genetic material probably deserves the name.

Genetic mutations occur in all organisms. But since large-scale mutations -- such as the random insertion of large DNA sequences within or between genes -- are almost always bad for an organism, Lynch and University of Oregon computer scientist John Conery suggest the only way junk DNA can survive the streamlining force of natural selection is if natural selection's potency is weakened.

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Contact: David Bricker
brickerd@indiana.edu
812-856-9035
Indiana University
20-Nov-2003


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